U.S. patent number 7,023,952 [Application Number 10/262,486] was granted by the patent office on 2006-04-04 for mechanical damper for air pad instability.
This patent grant is currently assigned to Koninklijke Philips Electronics. Invention is credited to William C. Brunnett.
United States Patent |
7,023,952 |
Brunnett |
April 4, 2006 |
Mechanical damper for air pad instability
Abstract
In a diagnostic imaging apparatus, a stationary gantry (24) and
a rotating gantry (22) are interfaced by a plurality of air bearing
elements (40). Lower air bearing elements bear the weight of the
rotating gantry (22) which induce air hammering phenomena at a
characteristic vibration frequency. To counteract the air
hammering, a damping assembly (44) is mounted to at least one lower
bearing element (40). The damping assembly (44) includes a damping
mass (46) and an elastomeric connector (48) that are tuned to a
frequency near the air hammer frequency to absorb the vibrational
energy and damp the air hammer vibrations.
Inventors: |
Brunnett; William C. (Concord,
OH) |
Assignee: |
Koninklijke Philips Electronics
(Eindhoven, NL)
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Family
ID: |
32030230 |
Appl.
No.: |
10/262,486 |
Filed: |
October 1, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040062356 A1 |
Apr 1, 2004 |
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Current U.S.
Class: |
378/15;
378/4 |
Current CPC
Class: |
A61B
6/035 (20130101); F16C 32/0618 (20130101); F16C
32/0666 (20130101); F16C 32/067 (20130101); F16C
2316/10 (20130101); F16C 2300/14 (20130101) |
Current International
Class: |
A61B
6/03 (20060101) |
Field of
Search: |
;378/4,15,193,197 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 095 620 |
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May 2001 |
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EP |
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1 095620 |
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May 2001 |
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EP |
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05155263 |
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Jun 1993 |
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JP |
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5-269124 |
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Oct 1993 |
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JP |
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WO 01/07899 |
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Feb 2001 |
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WO |
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WO 02/24072 |
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Mar 2002 |
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WO |
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Primary Examiner: Glick; Edward J.
Assistant Examiner: Kao; Chih-Cheng Glen
Claims
Having thus described the preferred embodiments, the invention is
now claimed to be:
1. A diagnostic scanning apparatus comprising: a first, rotating
gantry; a second, stationary gantry; an imaging region defined by a
bore in the first gantry; an air bearing system including a
plurality of bearing elements, at least one of the elements
exhibiting mechanical disturbance; a mechanical damper mounted
adjacent at least one bearing element to dampen the mechanical
disturbance, the mechanical damper being not directly connected to
the stationary gantry.
2. The diagnostic scanning apparatus as set forth in claim 1,
wherein the mechanical disturbance is air hammering in a direction
perpendicular to a direction of force applied by the bearing
element to the rotating gantry.
3. The diagnostic scanning apparatus as set forth in claim 2,
wherein the mechanical damper is attached to the bearing element at
a point where the mechanical disturbance has the greatest
amplitude.
4. The diagnostic scanning apparatus as set forth in claim 2,
further including: a source of x-rays mounted to the rotating
gantry for rotation therewith; a detector array for detecting
x-rays originating from the source after they pass through the
imaging region; a reconstruction processor for reconstructing the
detected x-rays into an image representation.
5. The diagnostic scanning apparatus as set forth in claim 1
wherein the mechanical damper is tuned to a frequency near a
characteristic vibration frequency of the mechanical
disturbance.
6. A diagnostic scanning apparatus comprising: first and second
gantries that define an imaging region; an air bearing system that
connects the first and second gantries, the air bearing system
including a plurality of bearing elements, at least one of the
elements exhibiting mechanical disturbance; a damping mass for
providing momentum opposite to momentum of the mechanical
disturbance; and, an elastomeric connector that connects the
damping mass to one of the air bearing elements and absorbs energy
from the mechanical disturbance.
7. The diagnostic scanning apparatus as set forth in claim 6,
wherein the elastomeric connector includes a plurality of rods
connecting the bearing element to the damping mass.
8. The diagnostic scanning apparatus as set forth in claim 7,
wherein the rods are each fastened to the bearing element with a
threaded fastener.
9. The diagnostic scanning apparatus as set forth in claim 6,
wherein the elastomeric connector is one of Polyurethane and
Urethane.
10. The diagnostic scanning apparatus as set forth in claim 6,
wherein the elastomeric connector and the damping mass are tuned to
a frequency slightly less than a resonance frequency of the
mechanical disturbance.
11. A method of diagnostic scanning comprising: rotating a rotating
gantry about a subject in an examination region; supporting the
rotating gantry on a plurality of bearing elements supported by a
stationary gantry, at least one of which bearing elements tends to
vibrate at a characteristic frequency; mechanically damping the
vibrating bearing element by mounting mass to the at least one
bearing element with a resilient connector, the mass and resilient
connector being not directly connected to the stationary
gantry.
12. The method as set forth in claim 11, further including: tuning
the mass and the resilient connector to a damping frequency near
the characteristic vibration frequency.
13. The method as set forth in claim 12, wherein the damping
frequency is slightly less than the characteristic vibration
frequency.
14. A method of diagnostic scanning comprising: rotating a rotating
gantry about a subject in an examination region; supporting the
rotating gantry on a plurality of air bearing elements which are
supported by a stationary gantry, at least one of which bearing
elements tends to vibrate at a characteristic frequency;
mechanically damping the vibrating bearing element with a damping
element which is tuned to the characteristic frequency and not
directly connected to the stationary gantry.
15. The method as set forth in claim 14, wherein the vibration of
the air bearing element causes air hammering whose energy is
absorbed by the mechanical damping to reduce the air hammering
motion.
16. The method as set forth in claim 14, wherein the mechanical
damping step includes: absorbing energy with an elastomeric element
and mass that are attached to the air bearing element.
17. The method as set forth in claim 14, further including:
emitting x-rays through the examination region; detecting the
x-rays after they pass through the examination region;
reconstructing the detected x-rays into an image
representation.
18. A CT scanner comprising: a stationary gantry; a plurality of
air bearing pads mounted to the stationary gantry; a mass mounted
to at least a lower one of the air bearing pads with a resilient
element, the mass being not directly connected to the stationary
gantry; a rotating gantry having an annular race supported by the
air bearing pads; an x-ray source mounted to the rotating gantry;
an array of x-ray detectors mounted to one of the rotating and
stationary gantries for receiving x-rays from the x-ray source; a
reconstruction processor connected with the detector array for
reconstructing outputs of the detector array into an electronic
image representation.
Description
BACKGROUND OF THE INVENTION
The present invention relates to medical imaging arts. In
particular, it relates to a rotating gantry such as those found in
3.sup.rd and 4.sup.th generation CT scanners, and will be described
with particular reference thereto. However, the invention will also
find application in conjunction with other systems, such as nuclear
cameras that use rotating gantries, and is not limited to the
aforementioned application.
Typically, 3.sup.rd and 4.sup.th generation CT scanners are
equipped with mechanical ball or roller bearing systems. Because
there is physical contact between the bearings and the rotating
gantry, there is friction and wear that occurs over usage of the
scanner. Additionally, functional speeds of the rotating gantry are
limited by mechanical the bearings.
In an effort to overcome the limitations of mechanical bearing
systems for such medical imagers, fluid bearing systems are being
used. Some fluid bearing systems include porous bearing pads that
fit snugly to bearing races of the rotating gantry. When the
bearing system is charged, a micro-thin layer of fluid is ejected
from the porous bearing pads between the pads and the bearing
races. This provides a virtually frictionless support for the
rotating gantry.
In such a system, the bearing pads that bear the weight of the
rotating gantry typically exhibit a phenomenon called air
hammering. Because of the shape of the bearing pad, and the stress
exerted on the bearing due to the weight of the gantry, minor
pressure inconsistencies of the bearing can result in rotational
wobbling of the bearing pads. Air hammering can lead to premature
wear of the bearing pads around the edges, where they frequently
come into contact with the race, premature wear of the race for the
same reason, and excess noise from the scanner.
In attempts to counteract the vibrational disturbances due to air
hammering, previous systems have included conventional spring and
damper means. A damper is attached to a rigid body, such as the
stationary gantry. This damper is then attached to the bearing
element. This is a cumbersome setup, requiring amounts of space
that might not be available in a cramped gantry system.
Additionally, such dampers are removed from the bearing elements if
the bearing elements are removed from the stationary gantry.
The present invention contemplates an improved apparatus and
method, which overcomes the aforementioned limitations and
others.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a diagnostic
imaging apparatus is provided. First and second gantries define an
imaging region. A bearing system includes a plurality of bearing
elements, at least one of which exhibits a mechanical disturbance.
A mechanical damper is mounted with the at least one bearing
element that dampens the mechanical vibrations.
According to another aspect of the present invention, a method of
diagnostic scanning is provided. A gantry is rotated about a
subject in an examination region. The rotating gantry is supported
by a plurality of bearing elements, at least one of which tends to
vibrate at a characteristic frequency. The vibrations of the
bearing element are dampened.
According to another aspect of the present invention, a CT scanner
is provided. A plurality of bearing pads are movably mounted to a
stationary gantry for supporting a rotating gantry. A mass is
mounted to at least one of the bearing elements by a resilient
element. An x-ray source is mounted to one of the gantries. An
array of detectors receives x-rays from the x-ray source. A
reconstruction processor reconstructs outputs of the detector array
into an electronic image representation.
One advantage of the present invention resides in reduced wear of
component parts of a fluid bearing.
Another advantage resides in reduced noise of a fluid bearing
system.
Another advantage resides in increased rotational potential of the
rotating gantry.
Another advantage resides in a damper that does not require
attachment to a fixed body.
Numerous additional advantages and benefits of the present
invention will become apparent to those of ordinary skill in the
art upon reading the following detailed description of the
preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may take form in various components and arrangements
of components, and in various steps and arrangements of steps. The
drawings are only for the purpose of illustrating preferred
embodiments and are not to be construed as limiting the
invention.
FIG. 1 is a diagrammatic illustration of a computed tomography
scanner, in accordance with the present invention;
FIG. 2 is a perspective view of a lower air bearing element
including an inertial damper, in accordance with the present
invention;
FIG. 3 is an alternate embodiment of the inertial damper of FIG. 2
with a single entity connector;
FIG. 4 is a schematic representation of the air bearing element of
FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to FIG. 1, a CT scanner 10 includes a subject couch
12 for moving a subject disposed thereon into and out of an imaging
region 14. X-rays from an x-ray source 16 are shaped and collimated
into a fan beam, pass through the imaging region 14 and are
detected by a detector assembly 20 on the far side of the imaging
region 14. In the illustrated 3.sup.rd generation embodiment, the
source 16 rotates concurrently with the detector assembly 20,
always remaining 180.degree. around the imaging region 14 from the
detector assembly 20 as it rotates around an axis A. Alternately, a
stationary ring of individual detectors on a stationary gantry 22
can replace the detector array 20, as in a 4.sup.th generation CT
scanner.
Intensities of detected x-rays are collected in a data memory 30 as
a rotating gantry 24 rotates the x-ray source 16 about the subject.
As the data is collected, a reconstruction processor 32 applies a
convolution and backprojection algorithm, or other suitable
reconstruction technique, to the collected data, forming an image
representation. The image representation(s) are stored in an image
memory 34. A video processor 36 withdraws selected portions of the
image representations and formats them for viewing on a human
readable monitor 38 such as a CRT monitor, active matrix monitor,
LCD display, or the like.
The first, rotating gantry 24 is disposed within the second,
stationary gantry 22. The x-ray source 16 and the detector array 20
are mounted on the rotating gantry 24. Radial air bearing elements
40 are attached to the stationary gantry 22 by ball joints and abut
against a bearing race 42 of the rotating gantry 24. As discussed
in the background, the weight of the rotating gantry 24 compresses
the air bearing between the gantry 24 and the lower bearing
elements 40 such that the phenomenon of air hammering tends to
occur. Inertial dampers 44 are attached to the lower bearing
elements 40 to dampen the movement of the air pads.
With reference to FIG. 2, the lower air bearing elements 40 exhibit
air hammering, that is rotation indicated by the arrow B. The
inertial damper 44 includes a damper mass 46 and a resilient,
dampening connector 48. The mass 46 is sized proportionately to the
bearing element 40 to have the greatest dampening effect at the
resonant frequency of the hammering disturbance. Additionally, the
farther away from the axis of rotation of the disturbance, (in this
case the connection of the bearing element 40 to the stationary
gantry 22) the more effective the damper will be. Preferably, the
damper 44 is positioned at an extremity of the air bearing element
40, and can alternately be placed on a side of the bearing element
40 rather than the top as shown in FIGS. 1, 2, and 3. The mass 46
is connected to the bearing element 40 by the resilient connector
48. The connector 48 is made of high dampening elastomeric polymer.
Suitable elastomers include, but are not limited to, Polyurethane
and Urethane. The connector 48 taken in conjunction with the
inertial mass 46 dampens the majority of the inherent air
hammering.
The connector 48 acts as both a spring and a damper. The damping
frequency of the damper mass 46 and the connector 48 is tuned to a
frequency slightly less than the instability frequency of the
bearing element 40. As the bearing element 40 starts to excite at
its instability frequency, the bearing element 40 starts to excite
the damper mass 46. The connector 48 absorbs some of that energy,
thus reducing the motion of the bearing element 40. In the
preferred embodiment, the connector includes a plurality of pins,
each secured to the air bearing element 40 by a threaded connector.
Alternately, and as shown in FIG. 3, the connector 48 can be a
single elastomeric mass. It is to be understood that other
connectors and adhesives are also contemplated.
With reference to FIG. 4, the preferred embodiment can be seen in a
mechanical schematic. The motion of the bearing element 40 and
damper 44 system can be described by the following differential
equations:
.function..times..times..times..times..times..times..times..times.
##EQU00001## ##EQU00001.2## .function..function..function.
##EQU00001.3## where z is the position along one axis of the
bearing element 40 and z.sub.2 is the position along the same axis
of the damper mass 46, L is the distance between a center of
rotation of the bearing element 40 and an attachment point of the
damper 44, J.sub.p is the rotational displacement of the bearing
element 40, K.sub.p is the rotational spring constant of the air
bearing itself, K.sub.2 is the linear spring constant of the
elastomeric connector 48, B.sub.p is the damping coefficient of an
attachment of the bearing element 40 to the stationary gantry 22,
B.sub.2 is the damping coefficient of the connector 48, T.sub.i is
the torque on the air bearing element 40, g.sub.c is the
gravitational conversion constant and M is the mass of the damping
mass 48. From these equations, assuming that K.sub.p, J.sub.p,
T.sub.i, L, and B.sub.p are known or measured and fixed,
appropriate values for B.sub.2, K.sub.2 and M can be
determined.
The invention has been described with reference to the preferred
embodiments. Obviously, modifications and alterations will occur to
others upon reading and understanding the preceding detailed
description. It is intended that the invention be construed as
including all such modifications and alterations insofar as they
come within the scope of the appended claims or the equivalents
thereof.
* * * * *